You've heard time and again that "256-bit" encryption is the bee's knees when it comes to security. But what does that even mean? Encryption is a topic that can be quite complex, though this educational video from 3Blue1Brown does an excellent job of visualising how hard 256-bit is to "crack".
A 256-bit key — or hash — is simply a fixed-length representation of a variable-length message. This could be some text, or a password — anything really. This hash can then be used to quickly and easily verify the integrity of the original message.
For the video, 3Blue1Brown uses the example of cryptocurrency hashes, as this clip is a follow-up to an earlier clip on bitcoin. Once the basics are explained, 3Blue1Brown describes the sort of computational power need to guess a hash using random inputs.
Once we enter the realm of the "GigaGalactic Super Computer", it quickly becomes obvious that brute-forcing a 256-bit key would take a very, very long time. Even replacing 3Blue1Brown's GPU-packed example computer with dedicated hardware such as application-specific integrated circuits (ASIC), capable of per-second output in tera-hashes, you — and the universe — would be very, very dead before you'd see a result.
It's hard to say "never" when it comes to technology, but I think it's safe to say that 256-bit is rather good for our current needs.